add a default constructor for the TypeId class

src/core/object.cc

@@ -365,6 +365,10 @@ TypeIdTraceResolver::TraceAll (std::ostream &os, const TraceContext &context)
  *         The TypeId class
  *********************************************************************/
 
+TypeId::TypeId ()
+  : m_tid (0)
+{}
+
 TypeId::TypeId (std::string name)
 {
   uint16_t uid = Singleton<IidManager>::Get ()->AllocateUid (name);

src/core/object.h

@@ -233,6 +233,8 @@ public:
                        uint32_t flags,
                        Ptr<const ParamSpec> spec);
 
+  // construct an invalid TypeId.
+  TypeId ();
   ~TypeId ();
 private:
   friend class Object;